This invention relates to optical cable connectors which maintain polarization of optical beams transported between the optical fibers themselves or between the fibers and optical devices such as a laser diode.
Among the various optical fibers, there are those fibers which are capable of maintaining the polarization of an optical beam. Such polarization maintaining fiber (referred to as PMFs hereinafter) has a feature that they maintain the direction of polarization of a linearly polarized beam entered into an optical fiber (hereinafter an optical fiber is referred simply as a fiber) in the direction of orientation of its principal axis.
A parameter for evaluating the performance of such PMFs is the extinction ratio between the intensity of a linearly polarized beam at the output end in the principal axis direction and the intensity of the beam in the direction perpendicular to the principal axis. For example, in a case of a short cable of about 10 m, a value of the extinction ratio between -40 to -50 dB has been realized.
In such cases, the use of zirconia ferrules is disclosed for example, in a Japanese Patent Application, First Publication No. S61-170709, for easy connection and disconnection (referred to as mating) between PMFs themselves and between PMFs and optical devices, such as laser diode, which generate polarized beams. As shown in FIG. 1, such a zirconia ferrule 1 (hereinafter ferrule refers to zirconia ferrule,) is made of a cylindrical rod-shaped insertion portion 3 made of a unit body of zirconia, and having a through hole 2 for passing a fiber of a diameter which is slightly larger than the fiber, into a metal stem 5 provided with a keyway 4. A PMF is fixed firmly in the center of the ferrule 1, which is assembled into the plug housing of a connector. Optical connection is made by coupling two faces of ferrules together, which aligns the ends of two ferrules 1, when the end portion of ferrule 1 is inserted into the alignment sleeve of a precision inner diameter provided in an adaptor. The two end faces of the ferrules 1 are pressed tightly by means of the compression force of springs provided in the plug housing.
An example of the connectors of such a design is a threaded coupling FC-type optical cable connector (F01 Type Connectors for Optical Fiber Codes specified in JIS C 5970). This type of connector is composed of a cylindrical plug housing which houses the above-mentioned ferrule 1, and the optical connection is made by inserting this plug housing into an adaptor, and by completing the connection by means of a screw-on type coupling nut which tightens the plug housing onto the adaptor. Another type is a push-pull coupling SC-type optical cable connector (F04 Type Connectors for Optical Fiber Codes specified in JIS C 5973). This type is composed of a rectangular plug housing which houses the above-mentioned ferrule 1, and the plug housing is inserted and set in the adaptor.
However, in making such optical connections between the PMFs or between PMFs and optical devices, it is necessary to align the principal axis of each of the two fibers, i.e. to rotation align the axial angles.
For this reason, in the above mentioned FC-type connector, after the ferrule 1 has been assembled into the plug housing, an adjustment key is attached to the plug housing, and by rotating the adjustment key, the angular orientation between the adjustment key and the principal axis of the PMF is obtained. However, upon inserting a plug into an adaptor, the plug is rotated to align the key on the plug with the keyway in the adaptor after the ferrule has been set into the plug housing, therefore, there was a problem that the ferrule 1 is subjected to a twisting torque. Further, in this FC-type connector, when the coupling nut is tightened, there is a rotation, although slight, of the plug housing with respect to the adaptor, resulting in an application of additional torque on the ferrule 1. When the plug housing and the adaptor are put through repeated mating cycles, the principal axes of the two fibers to be connected became shifted, and produced random variations in the extinction ratio, as illustrated FIG. 2.
On the other hand, when fixing the PMF in the ferrule 1 in the above mentioned SC connector, the axial angle between the ferrule 1 and the keyway 4 are preadjusted, and the pre-adjusted ferrule 1 is set into the adaptor. Further, the direction of insertion of the adaptor of the SC-type connector involves only a movement in the longitudinal direction, and the alignment of the axial angle with the adaptor is carried out before the ferrule is inserted into the alignment sleeve, therefore, there is no torque imposed on the ferrule due to insertion of the plug. It follows, therefore, that even after repeated mating cycles, it is possible to retain a relatively constant extinction ratio as shown in FIG. 2.
The methods of adjusting the principal axis of the fiber with respect to the ferrule and fixing the fiber on the ferrule are disclosed, for example, in Japanese Patent Application First Publication No. H1-232308.
FIG. 3 shows the assembling device used in the above mentioned method, in which the numeral 11 is a ferrule locking part which holds the ferrule in place, 12 is a fiber retention part which holds the PMF 13 and 14 is a rotation means for rotating the fiber fixing part.
The assembling order is, first, a fiber 13 whose end has been cleaved at right angles to the fiber axis is inserted into a ferrule 1, which is secured in the ferrule locking part 11 by engaging a key (not shown) into a keyway 4 provided on the ferrule 1.
Then, the coated optical fiber of the PMF 13 is fixed on the fiber retention part 12. Next, while observing the end face of the PMF 13 under a microscope, the PMF 13 is axially rotated by means of the rotation means 14 to set the PMF 13 in position. The rotation means 14 is locked in position, and the ferrule 1 is taken out first, and the ferrule i is filled with an adhesive, the locked PMF is re-inserted and the ferrule 1 is secured in position in the ferrule locking part 11. At this time, the key of the ferrule locking part 11 and keyway 4 of the ferrule 1 are engaged. The assembly is placed in an oven to harden the adhesive.
By following such a procedure, it is possible to bond the PMF 13 in the ferrule 1 while maintaining the certain required axial angle between the principal axis of the PMF and the ferrule key 4. However, the above mentioned assembling procedure presented the following problems.
(1) During the adhesive curing period lasting approximately two hours, the assembling device is monopolized by one ferrule, resulting in extreme inefficiency of production.
(2) Unlike the ordinary single mode fibers, because PMFs retain large residual stresses in the fiber, even when the fiber is cleaved, it is difficult to obtain perpendicular fracture surface, and consequently, it is difficult to identify the location of stress-applying parts.
(3) Because the coated section of the PMF 13 is held in the fiber retention part 12, the fiber is able to rotate in the coated part during heating, resulting in a deviation of the preset axial angle.
The assembling cost becomes high due to the above factor (1), and the setting precision is degraded by the factors (2) and (3).